yes, another w123 tach thread

[kuene]

OK, this isn't about fixing the tach with a cig butt trick.

I have 2 problems with my tach. I'm not so much interested in #1, but I'm mentioning it because it certainly may be relevant.

1) It's works intermittently - about 50% time, it just works when I start up the car to when I shut down. Occasionally, it will stop working on a drive, and other times it doesn't' work at the start of driving, but it just kicks on randomly at some point. Sometimes, it just doesn't work period. There is absolutely ZERO touching of the tach amp unit at all during any of these on/off cases. I've replaced the original amp with a known working one (at least it was when I replaced it a year ago), and I've cleaned and inspected the pickup/sender (though haven't thoroughly tested the cable itself).

2) When the tach is working (about 50% of the time) it doesn't really registered ANY rpms under 1000. So if I'm idling, the needle is just at rest. I have to have my toe on the pedal a little bit, and it immediately bounces up to 1k or above. When I'm driving around in various gears, and watching it during shifts, it seems to be reacting normally.

Anybody have any clue why the tach just seems to ignore anything below 1k? Is that threshold related to some specific component in the tach amp?